And (with the exception of carbon dating) how do we know how old the universe is?
*Cough* *cough* *sputter*
Peeta, how much science schooling have you had? Are you in the least familiar with carbon dating? With radiometric dating? With half-lives?
Let's say that you need to weigh something. What kind of a scale would you use? Simply put, if you were really serious about making that measurement, then you would have to pick the appropriate scale.
OK, I've been trained as an electronics technician, in which one of our tools is a voltmeter. If you pick up a multi-meter (measures voltage, current, and resistance over multiple ranges) to measure a certain voltage, which range do you use? Ignoring standard procedure in which we don't peg the D'Arsonval meter movement which would destroy it (ultimately within a physical multimeter it all boils down to measuring current, though things are different in a digital multimeter), we need to choose the right range to make our measurement. Pick a range too low and the reading will read at the max, but if you pick a range too high then the reading will be too low to measure accurately. The same idea applies to weighing something and having to know to use a bathroom scale, a truck scale, or an apothecary's scale.
Different isotopes have different decay rates, different half-lives. Some are longer than others, much much longer. As a result, the isotopes with longer half-lives can measure time for much longer than isotopes with much shorter half-lives. Uranium half-lives are very long, so their half-lives and their daughters' half-lives can be used to measure long periods of time, periods of time appropriate to measuring the age of the earth. Carbon-14's half-life is much shorter, such that the longest period of time that C-14 can measure is about 50,000 years.
So if you apply C-14 dating to something that is much older than 50,000 years, you will come up with the maximum reading of the C-14 scale, 50,000 years. Over the decades, I have watched one creationist after another parade some sample millions of years old that they had subjected to C-14 testing and came up with an age of 50,000 years. Duh? Kent Hovind, convicted on the federal level of fraud (OK, tax fraud, but he also has lied about everything else), had even submitted a completely fossilized specimen for C-14 testing and it came back aged at the maximum, 50,000 years, though in that YouTube clip, the narrator kept shouting, "But there's no f***ing carbon!"
Radio-carbon dating has absolutely nothing to do with determining the age of the earth. When a creationist tries to claim that it does, then that is immediate proof that that creationist has absolutely no idea what he's talking about. Kind of like when a creationist says, "But evolution is only a theory!"
But your question of how we do know how old the earth and the universe is is an appropriate and necessary question. The answers are there. You just need to be ready to learn them.
Consider The Age of the Earth by G. Brent Dalrymple of the US Geological Survey (1994). Most of the same have been published everywhere.
But in the meantime, you need to disabuse yourself of certain false ideas, such as carbon-dating having anything at all to do with the age of the earth, let alone the universe.
Edited by Adminnemooseus, : Fix message ID number, per RAZD in message 43 below.